Double density fuse bank for the laser break-link programming of an integrated circuit

ABSTRACT

A fuse bank for use in the laser break-link programming of an integrated circuit device. The fuse bank uses fuse elements with two ends that contain fusible regions proximate the first end and non-fusible regions proximate the second end. The fuse elements are aligned in alternately oriented parallel rows so that the first end of each fuse element is juxtaposed with the second end of any adjacent fuse element. By sequentially alternating the orientation of the fuse elements in the fuse bank, the fuse elements can be formed in a highly dense matter without bringing any two fusible regions too close to one another. Accordingly, a laser can be used to sever selected fusible regions without adversely effecting other fusible regions within the fuse bank. By alternating the orientations of sequential fuse elements, a fuse bank can be created that is twice as dense as single orientation fuse banks with only a 30% to 50% increase in size. The space savings on an integrated circuit enables more circuitry per unit space and promotes the further miniaturization of the integrated circuit device.

This is a divisional of application Ser. No. 08/780,242 filed Jan. 8,1997, now U.S. Pat. No. 5,773,869 which is a continuation of applicationSer. No. 08/365,641 filed Dec. 29, 1994, now abandoned.

FIELD OF THE INVENTION

The present invention relates to fuse banks used in conjunction withsemiconductor integrated circuits to program the integrated circuit andselectively remove defective circuits in a circuit bank. Morespecifically, the present invention relates to the structure of a fusebank, wherein the density of the fuse bank is increased to promoteminiaturization.

BACKGROUND OF THE INVENTION

Semiconductor integrated circuits contain large numbers of electronicdevices such as diodes and transistors built onto a single crystal orchip, often made of silicon. Since these devices are so small, theiroperational integrity is very susceptible to imperfections or impuritiesin the crystal. The failure of a single transistor in a circuit mayrender that circuit non-functional.

In order to circumvent this problem, the semiconductor industryregularly builds redundant circuits on the same chip. Therefore, if afaulty circuit is discovered during testing, the faulty circuit can bedisabled and its redundant circuit enabled. Often, this switching to aredundant circuit is accomplished by blowing certain fuses built intothe circuitry of the chip. Those fuses signal the location of thedefective element and enable a substitute element in a redundant circuitbank.

In the case of memory ICs, memory cells are usually arranged in rows andcolumns. Each memory cell is addressed by a particular row and column.By blowing the correct combination of fuses, circuitry which addressesthe faulty elements, can be isolated and replaced with circuitry whichaddressed a corresponding redundant element.

A common method of selectively blowing fuses in a fuse bank is by theuse of a laser, wherein energy from the laser is selectively directedtoward the various fuses. The laser melts the selected fuses andisolates the defective circuits. This process is commonly known as laserbreak-link programming or laser programming.

The process of laser break-link programming, however, does have itsdrawbacks. Primary among them is the required size of the fuse bank inorder to effectively use a laser. As a laser is directed against surfaceof a chip, it generally burns a round crater onto whatever surface thelaser strikes. When used to melt a fuse, the diameter of the crater madeby the laser must be wider than the fuse to ensure that the fuse iscompletely severed. However, the crater caused by the laser can not betoo wide or else the laser will melt fuses positioned next to thetargeted fuse. Consequently, prior art fuse banks need to bemanufactured with relatively large spaces between the various fuses inthe fuse bank. Due to semiconductor circuit manufacturing tolerances andlaser light tolerances, the typical fuse bank must have a spacing of atleast 4.5 μM to 5.4 μM in order for laser break-link programming to beused. This required spacing often causes multiple fuse banks to bepresent at various places on a integrated circuit chip. This consumes asignificant amount of space on the chip, limiting further miniaturizingof the integrated circuits.

The prior art is replete with different fuse bank structures thatattempt to reduce the size of fuse banks. Such prior art references areexemplified by U.S. Pat. No. 5,185,291 to Fisher, entitled METHOD OFMAKING SEVERABLE CONDUCTIVE PATH IN AN INTEGRATED-CIRCUIT DEVICE; U.S.Pat. No. 4,910,418 to Graham et al. entitled SEMICONDUCTOR FUSEPROGRAMMABLE ARRAY STRUCTURE; U.S. Pat. No. 4,935,801 to McClure et al.entitled METALLIC FUSE WITH OPTICALLY ABSORPTIVE LAYER; and U.S. Pat.No. 5,025,300 to Billing et al, entitled INTEGRATED CIRCUITS HAVINGIMPROVED FUSIBLE LINKS. Such prior art references provide structuresthat absorb laser radiation and increase the effectiveness of the laser.However, such prior art systems are still limited by the size of thelaser, wherein the spacing between fusible links must be large enough toaccommodate the diameter of the laser beam.

It is therefor an objective of the present invention to provide a fusebank with an increased density of fusible links, wherein the distancebetween separate elements in the fuse bank is less than the diameter ofthe laser beam used to sever those elements.

It is a further objective of the present invention to increase thedensity of a fuse bank by 100% while increasing the size of the fusebank by less than 50%.

SUMMARY OF THE INVENTION

The present invention is a fuse bank for use in the laser break-linkprogramming of an integrated circuit device. The fuse bank uses fuseelements with two ends that contain fusible regions proximate the firstend and non-fusible regions proximate the second end. The fuse elementsare aligned in alternately oriented parallel rows so that the first endof each fuse element is juxtaposed with the second end of any adjacentfuse element. By sequentially alternating the orientation of the fuseelements in the fuse bank, the fuse elements can be formed in a highlydense matter without bringing any two fusible regions too close to oneanother. Accordingly, a laser can be used to sever selected fusibleregions without adversely effecting other fusible regions within thefuse bank.

By alternating the orientations of sequential fuse elements, a fuse bankcan be created that is twice as dense as single orientation fuse bankswith only a 30% to 50% increase in size. The space savings on anintegrated circuit enables more circuitry per unit space and promotesthe further miniaturization of the integrated circuit device.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully understood by referring to thefollowing detailed description, the above background information and theclaims appended hereto, when considered in connection with theaccompanying drawings, wherein:

FIG. 1 shows two prior art fuse banks used in conjunction with twoseparate integrated circuit arrays;

FIG. 2 shows a preferred embodiment of the present invention fuse bankin conjunction with two separate integrated circuit arrays; and

FIG. 3 shows an enlarged segment of FIG. 2 displaying the interactionbetween a laser beam and the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, there is shown two prior art fuse banks 10 used toprogram two integrated circuit (IC) arrays 12. Each of the fuse banks 10includes a plurality of fusible elements 14 made of polycrystallinesilicon, tungsten silicide or a similar conductor. The fusible element14 have reduced regions 16 which are linearly aligned. The X'ssuperimposed over the reduced region 16 show the position where thecenter of the laser beam is aimed during laser break-link programming.Depending upon the manufacturing method used to make the fuse banks 10,the IC application and the laser used during laser break-linkprogramming, the distance D in between the various fusible elements 14is typically between 4.5 μM and 5.4 μM. As can be seen, in order toprovide laser break-link programming to two adjacent IC arrays 12, roomon the chip must be made for the two fuse banks 10 and the wiringleading to the two fuse banks 10. In a typical application, a singleprior art fuse bank would have a width W of approximately ten microns.

Referring to FIG. 2, the present invention fuse bank 20 is shown inconjunction with two IC arrays 12. As can be seen, a single bank 20supports the two IC arrays 12. In FIG. 2, the fuse bank 20 is comprisedof a series of compound fuse elements 22. Each compound fuse element 22has a reduced region 24 of readily fusible material such as that used inthe prior art. Target areas 26 on the reduced region 24 may have anoptically absorptive layer or other prior art structure that promotesthe efficiency of a laser in severing the fuse element. The target areas26 corresponds to the position of where the center of the laser beam isaimed during laser break-link programming.

Conductive elements 30 are coupled to the reduced region 24 of each ofthe fuse elements 22. The conductive elements 30 are made of anyconductive material that is resistant to the laser radiation at theintensity needed to sever the target areas 26. In order to be resistantto the laser radiation, the conductive elements 30 may be much thickerthan the target areas 26. Alternatively, the conductive elements 30 maybe made of a reflective material such as gold, tungsten or anothermetal. Alternatively, the conductive elements 30 may be coated with areflective material that does not absorb much of the laser's energy.Coatings and materials capable of resisting laser energy such astungsten, gold and the like are known and used in the manufacture ofsemiconductor circuits.

In the present invention fuse bank 20, the various target areas 26 ofthe fuse elements 22 do not align in a single column. Rather, theorientation of fuse elements 22 is staggered wherein every other fuseelement faces in a first direction and the interposed fuse elements facea second opposite direction. In this orientation only one fuse bank 20is needed to support two IC arrays 12, wherein the IC arrays aredisposed on either side of the fuse bank 20. The distance D1 betweentarget areas 26 on each adjacent fuse elements 22 is now within therequired 4.5 μM to 5.4 μM range, as is the distance D2 between thetarget areas 26 of every alternating fuse element. Since the variousfuse elements 22 are stacked twice as dense as in the prior art, theactual distance or pitch D3 in between adjacent fuse elements 22 isbetween, 2.2 μM and 2.7 μM, half of that available in the prior art.

By alternating the orientation of every other fuse element 22 in thefuse bank 20, the target areas 26 align is two distinct rows. Thisconfiguration adds approximately 30% to 50% more width W1 to the fusebank 20 as compared to prior art fuse banks. However, the presentinvention fuse bank 20 has twice as many fuses per unit length L than isavailable in the prior art. Consequently, a 100% increase in fusedensity can be had for a 30% to 50% increase in size. Further sizereductions are also provided by the elimination of the lead wiringneeded to join two separate fuse banks in the prior art.

Referring to FIG. 3, it can be seen that during laser break-linkprogramming, laser light, represented by circle 40, is directed towardthe target area 26 of one of the fuse elements 22. The laser light 40partially overlaps the conductive elements 30 of the two adjacent fuseelements 22a, 22b. However, since the target area 26 of the center fuseelement 22 is made of a material that absorbs laser radiation, the lasereasily severs the target area 26. The two conductive elements 30,however, are reflective to laser radiation and remain intact while thetarget area 26 is severed. The staggered configuration thereby preventsany two target areas from being exposed to the same pulse of laserlight. This enables the fuse bank 20 to be manufactured in a much densermanner than previously available.

The present invention, as previously described, can be used to make fusebanks more dense. However, by keeping the same density (fusible elementsper unit length), the present invention can be used to increase thespace in between the fusible elements within the fuse bank. With morespace available in between fusible elements, a wet etch process can beused to sever the fusible elements instead of a laser. Wet etchprocesses typically are less complex and less costly than laserbreak-link processes, thereby providing a cost and labor savings duringmanufacture.

It will be understood that the specific embodiments of the presentinvention described herein are merely exemplary and are present toexpress the best mode of the invention. However, a person skilled in theart may make many variations and modifications to the describedembodiments by using functionally equivalent components and processes.All such variations, modifications and alternate embodiments areintended to be considered within the scope of the invention as stated inthe following claims.

What is claimed is:
 1. A fuse bank for use in the laser programming ofan integrated circuit, comprising:a plurality of fusible elements eachhaving a first end and a second end wherein each fusible elementincludes a fusible region, proximate said first end, configured to besevered by a predetermined exposure of laser radiation and, anon-fusible region, proximate said second end, configured to beunsevered by said predetermined exposure to laser radiation; whereinsaid fusible elements are alternately aligned in substantially parallel,rows so that the first end of a first one of the fusible elements in oneof the rows, is juxtaposed with, and disposed between, the second endsof a pair of the fusible elements disposed in adjacent ones of the rowsof fusible elements.
 2. The fuse bank according to claim 1, wherein theparallel rows of fusible elements are disposed between 2.2 μM and 2.7 μMapart.
 3. The fuse bank according to claim 1, wherein said fusibleregion includes a target area adapted to absorb laser radiation.
 4. Thefuse bank according to claim 1, wherein each said fusible region isseparated by a distance of at least 4.5 μM.
 5. The fuse bank accordingto claim 1, wherein said nonfusible region includes a metal that issubstantially reflective to said predetermined exposure of laserradiation.
 6. The fuse bank recited in claim 1 wherein the first end ofsuch first one of the fusible elements is laterally offset from thefirst ends of said pair of fusible elements.
 7. The fuse bank recited inclaim 1 wherein the first ends are staggered from row to row.